Editing Bayer process (section)

== Process == 
[[File:Bayer-process-en.svg|thumb|right|500 px|The Bayer [[process flow diagram]].]]
Bauxite ore is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. The aluminium compounds in the bauxite may be present as [[gibbsite]] (Al(OH)<sub>3</sub>), [[boehmite|böhmite]] (γ-AlO(OH)) or [[diaspore]] (α-AlO(OH)); the different forms of the aluminium component and the impurities dictate the extraction conditions. Aluminium oxides and hydroxides are [[amphoteric]], meaning that they are both acidic and basic.  The solubility of Al(III) in water is very low but increases substantially at either high or low pH.  In the Bayer process, bauxite ore is heated in a [[pressure vessel]] along with a [[sodium hydroxide]] solution (caustic soda) at a temperature of {{convert|150|to|200|°C}}. At these temperatures, the [[aluminium]] is dissolved as [[sodium aluminate]] (primarily [Al(OH)<sub>4</sub>]<sup>−</sup>) in an extraction process. After separation of the residue by filtering, gibbsite is precipitated when the liquid is cooled and then [[Seed crystal|seeded]] with fine-grained aluminium hydroxide crystals from previous extractions.  The precipitation may take several days without addition of seed crystals.<ref name="Grocott99">{{cite journal |last1=Hind |first1=Andrew R. |last2=Bhargava |first2=Suresh K. |last3=Grocott |first3=Stephen C. |title=The surface chemistry of Bayer process solids: a review |journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects |date=January 1999 |volume=146 |issue=1–3 |pages=359–374 |doi=10.1016/S0927-7757(98)00798-5}}</ref>

The extraction process (''digestion'') converts the aluminium oxide in the ore to soluble sodium aluminate, NaAlO<sub>2</sub>, according to the [[chemical equation]]:

:Al(OH)<sub>3</sub>+  NaOH   →   NaAlO<sub>2</sub>  +  2 H<sub>2</sub>O

This treatment also dissolves silica, forming sodium silicate :

:2 NaOH + SiO<sub>2</sub>   →   Na<sub>2</sub>SiO<sub>3</sub> + H<sub>2</sub>O 

The other components of Bauxite, however, do not dissolve. Sometimes{{when|date=October 2019}} [[lime (material)|lime]] is added at this stage to precipitate the silica as [[calcium silicate]]. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a [[flocculant]] such as [[starch]], to remove the fine particles. The undissolved waste after the aluminium compounds are extracted, [[bauxite tailings]], contains [[iron oxides]], [[silica]], [[calcia]], [[Titanium dioxide|titania]] and some unreacted alumina. The original process was that the [[alkali]]ne solution was cooled and treated by bubbling carbon dioxide through it, a method by which aluminium hydroxide [[precipitation (chemistry)|precipitates]]:

:2 NaAlO<sub>2</sub> + 3 H<sub>2</sub>O + CO<sub>2</sub> → 2 Al(OH)<sub>3</sub> + [[sodium carbonate|Na<sub>2</sub>CO<sub>3</sub>]]

But later, this gave way to seeding the supersaturated solution with high-purity aluminium hydroxide (Al(OH)<sub>3</sub>) crystal, which eliminated the need for cooling the liquid and was more economically feasible:

:2 H<sub>2</sub>O + NaAlO<sub>2</sub> → Al(OH)<sub>3</sub> + NaOH

Some of the aluminium hydroxide produced is used in the manufacture of water treatment chemicals such as [[aluminium sulfate]], PAC ([[Aluminium chlorohydrate|Polyaluminium chloride]]) or sodium aluminate; a significant amount is also used as a filler in rubber and plastics as a fire retardant.  Some 90% of the gibbsite produced is converted into [[aluminium oxide]], Al<sub>2</sub>O<sub>3</sub>, by heating in [[rotary kiln]]s or fluid flash [[calciner]]s to a temperature of about {{convert|1470|K}}.

:2 [[Aluminium hydroxide|Al(OH)<sub>3</sub>]] → [[alumina|Al<sub>2</sub>O<sub>3</sub>]] + 3 [[water|H<sub>2</sub>O]]

The left-over, 'spent' sodium aluminate solution is then recycled. Apart from improving the economy of the process, recycling accumulates [[gallium]] and [[vanadium]] impurities in the liquors, so that they can be extracted profitably.

Organic impurities that accumulate during the precipitation of gibbsite may cause various problems, for example high levels of undesirable materials in the gibbsite, discoloration of the liquor and of the gibbsite, losses of the caustic material, and increased viscosity and density of the working fluid.

For bauxites having more than 10% silica, the Bayer process becomes uneconomic because of the formation of insoluble [[sodium aluminium silicate]], which reduces yield, so another process must be chosen.

{{convert|1.9|-|3.6|ST|t lb|order=out}} of bauxite (corresponding to about 90% of the alumina content of the bauxite) is required to produce {{convert|1|ST|t lb|order=out}} of aluminium oxide. This is due to a majority of the aluminium in the ore being dissolved in the process.<ref name="Grocott99" /> Energy consumption is between {{convert|7|to|21|GJ/t|kWh/lb}} (depending on process), of which most is thermal energy.<ref>{{cite book |doi=10.1007/978-3-319-48248-4_24 |author1=Alessio Angelo Scarsella, Sonia Noack, Edgar Gasafi, Cornelis Klett, Andreas Koschnick |title=Light Metals 2015 |chapter=Energy in Alumina Refining: Setting New Limits|year=2015 |pages=131–136 |isbn=978-3-319-48610-9 }}</ref><ref>{{cite web |title=Energy efficiency |url=http://bauxite.world-aluminium.org/refining/energy-efficiency/ |quote=energy required by the Bayer Process is very much dependent on the quality of the raw material .  average specific energy consumption is around 14.5 GJ per tonne of alumina, including electrical energy of around 150 kWh/t Al2O3}}</ref> Over 90% (95-96%) of the aluminium oxide produced is used in the [[Hall–Héroult process]] to produce aluminium.<ref>{{cite web|title=The Aluminum Smelting Process|url=http://www.aluminum-production.com/important_figures.html |website=Aluminum Production|publisher=aluminumproduction.com |access-date=12 April 2018}}</ref>